In a variety of environments, such as in process control systems, analog or digital signals must be transmitted between diverse sources and circuitry using those signals, while maintaining electrical (i.e., galvanic), isolation between the sources and the using circuitry. Isolation may be needed, for example, between analog sensors and amplifiers or other circuits which process sensor outputs; or between microcontrollers, on the one hand, and sensors or transducers which generate or use microcontroller input or output signals, on the other hand. Electrical isolation is intended, inter alia, to prevent extraneous transient signals, including common-mode transients, from inadvertently being processed as status or control information, or to protect equipment from shock hazards or to permit the equipment on each side of an isolation barrier to be operated at a different supply voltage, among other known objectives. One well-known method for achieving such isolation is to use optical isolators that convert input electrical signals to light levels or pulses generated by light emitting diodes (LEDs), and then to receive and convert the light signals back into electrical signals. Optical isolators present certain limitations, however: among other limitations, they are rather non-linear and their current transfer ratios can vary over a large range with input voltage, temperature and lifetime, presenting challenges to the user and generally making them unsuitable for accurate linear applications; they require significant space on a card or circuit board; they draw a large current; they do not operate well at high frequencies; and they are very inefficient. They also provide somewhat limited levels of common mode transient immunity. To achieve and common mode transient immunity, opto-electronic isolators have been made with some attempts at providing an electrostatic shield between the optical transmitter and the optical receiver. However, a conductive shield which provides a significant degree of common mode transient immunity improvement is not sufficiently transparent for use in this application.
One isolation amplifier avoiding the use of such optical couplers is U.S. Pat. No. 5,831,426 to Black et al, which shows a current determiner having an output at which representations of input currents are provided, having an input conductor for the input current and a current sensor supported on a substrate electrically isolated from one another but with the sensor positioned in the magnetic fields arising about the input conductor due to any input currents. The sensor extends along the substrate in a direction primarily perpendicular to the extent of the input conductor and is formed of at least a pair of thin-film ferromagnetic layers separated by a non-magnetic conductive layer. The sensor can be electrically connected to electronic circuitry formed in the substrate, including a nonlinearity adaptation circuit to provide representations of the input currents of increased accuracy despite nonlinearities in the current sensor, and can include further current sensors in bridge circuits. Another non-optical isolation amplifier, for use in a digital signaling environment, is described in U.S. Pat. No. 4,748,419 to Somerville. In that patent, an input data signal is differentiated to create a pair of differential signals that are each transmitted across high voltage capacitors to create differentiated spike signals for the differential input pair. Circuitry on the other side of the capacitive barrier has a differential amplifier, a pair of converters for comparing the amplified signal against high and low thresholds, and a set/reset flip-flop to restore the spikes created by the capacitors into a logic signal. In such a capacitively-coupled device, however, during a common mode transient event, the capacitors couple high, common-mode energy into the receiving circuit. As the rate of voltage change increases in that common-mode event, the current injected into the receiver increases. This current potentially can damage the receiving circuit and can trigger a faulty detection. Such capacitively coupled circuitry thus couples signals that should be rejected. The patent also mentions, without elaboration, that a transformer with a short R/L time constant can provide an isolation barrier, but such a differential approach is nonetheless undesirable because any mismatch in the non-magnetic (i.e., capacitive) coupling of the windings would cause a common-mode signal to appear as a difference signal.
In a coil-based isolator, an input signal is coupled (directly or indirectly) from an input node to a coil or coils which generate(s) a magnetic field(s) that is(are) coupled to a gavanically isolated field-receiving or field-sensing element(s). An output circuit(s) coupled to the field-receiving element(s) converts the received field variations to an output signal corresponding to the input signal. The coupling from input node to input-side coil(s) may be made through elements or driving circuits that drive the coil(s) with signals derived from the input signal and not the same as the input signal.
Typically, there are two classes of coil-based isolators, depending on the nature of the field-receiving elements. The field-receiving elements may be another coil(s) or a type of magneto-resistive (MR)/giant-magneto-resistive (GMR) element(s) (which are collectively referred to herein as MR elements unless the need appears to distinguish them). If the field-receiving element(s) is(are) a coil(s), then the field-generating and field-receiving coils form transformers. With respect to coil-based isolators, we shall use the terms “winding” and “coil” interchangeably.
In transformer-based isolators, a Faraday shield may be interposed between the primary and secondary windings. The input signal is referenced to a first ground, or reference potential, and the output signal is referenced to a second ground, or reference potential. The Faraday shield also is referenced to the second ground. Common mode transients are capacitively coupled from the field generator (i.e., primary winding) into the Faraday shield and therethrough to the second ground, instead of into the corresponding field-receiving elements (i.e., secondary windings). Further, two Faraday shields may be disposed in spaced relationship between the transformer windings. In such an arrangement, a first Faraday shield usually is at the first reference potential and a second Faraday shield usually is at the second reference potential.
The isolation barrier may be formed on one or two silicon die, as shown in Application Serial No. 838,520, and can be formed from one or more pairs of field-generating and field-receiving elements, in each case preferably creating a vertically stacked structural arrangement with a dielectric (and in the case of windings, a Faraday shield) between the field-generating and field-receiving elements, for electrical isolation.
Such, an isolator may be monolithically fabricated. As disclosed in U.S. patent application Ser. No. 09/838,520, either one die or two may be used. An embodiment is shown therein with a complete isolator formed monolithically on a single die.
With appropriate driver and receiver circuits, embodiments of the isolator are useful for either analog signals or digital signals. Exemplary driver and receiver circuits for each type of signal also were shown in said application, though other driver and receiver circuits may be used, of course.
A need exists, however, to reduce the size and cost of such coil-based isolators, without degrading performance and without requiring greater power consumption. The cost of such isolators is highly dependent on semiconductor wafer fabrication costs and wafer area requirements.